When will blood oxygen reading be available for Apple Watch?

It should be available to you now if you update your iPhone to iOS 18.6.1 and your Watch to watchOS 11.6.1. See the following An update on Blood Oxygen for Apple Watch in the U.S. - Apple

Note the following excerpt from that article:

Following this update, sensor data from the Blood Oxygen app on Apple Watch will be measured and calculated on the paired iPhone, and results can be viewed in the Respiratory section of the Health app. 

Also note that some people haven't been able to get it to work after updating. Some people have fixed that by running the ECG. Others have had to unpair and re-pair the watch. So far, I haven't seen anyone who couldn't get it working.

Repairing wasn’t needed for me, just restarting the watch and then taking an ECG

Kilgore-Trout wrote:

Repairing wasn’t needed for me, just restarting the watch and then taking an ECG

Good to know.